Nucleic acids were discovered in 1868 by the Swiss scientist F. Miescher.
In organisms, there are several types of nucleic acids that are found in various cell organelles - the nucleus, mitochondria, plastids.
Nucleic acids include DNA, i-RNA, t-RNA, r-RNA.
Deoxyribonucleic acid (DNA)
– a linear polymer in the form of a double helix formed by a pair of antiparallel complementary (corresponding to each other in configuration) chains. The spatial structure of the DNA molecule was modeled by American scientists James Watson and Francis Crick in 1953.
The monomers of DNA are nucleotides
.
Every nucleotide DNA consists of purine (A - adenine or G - guanine) or pyrimidine (T - thymine or C - cytosine) nitrogenous base, five carbon sugar– deoxyribose and phosphate group.
Nucleotides in a DNA molecule face each other with their nitrogenous bases and are paired together according to rules of complementarity: opposite adenine is thymine, opposite guanine is cytosine. The A – T pair is connected by two hydrogen bonds, and the G – C pair is connected by three. During the replication (doubling) of a DNA molecule, hydrogen bonds are broken and the chains separate, and a new DNA chain is synthesized on each of them. The backbone of DNA chains is formed by sugar phosphate residues.
The sequence of nucleotides in a DNA molecule determines its specificity, as well as the specificity of the body proteins that are encoded by this sequence. These sequences are individual for each type of organism and for individual individuals.
Example
:
The sequence of DNA nucleotides is given: CGA – TTA – CAA.
On messenger RNA (i-RNA), the chain HCU - AAU - GUU will be synthesized, resulting in a chain of amino acids: alanine - asparagine - valine.
When nucleotides in one of the triplets are replaced or rearranged, this triplet will encode a different amino acid, and, consequently, the protein encoded by this gene will change. Changes in the composition of nucleotides or their sequence are called mutation
.
Ribonucleic acid (RNA)
– a linear polymer consisting of a single chain of nucleotides. In RNA, the thymine nucleotide is replaced by uracil (U). Each RNA nucleotide contains a five-carbon sugar - ribose, one of four nitrogenous bases and a phosphoric acid residue.
RNA is synthesized in the nucleus. The process is called transcription
- this is the biosynthesis of RNA molecules on the corresponding sections of DNA; the first stage of the implementation of genetic information in a cell, during which the sequence of DNA nucleotides is “rewritten” into the nucleotide sequence of RNA.
RNA molecules are formed on a matrix, which is one of the DNA chains, the sequence of nucleotides in which determines the order of inclusion of ribonucleotides according to the principle of complementarity. RNA polymerase, moving along one of the DNA chains, connects nucleotides in the order determined by the matrix. The resulting RNA molecules are called transcripts.
Types of RNA.
Matrix or informational RNA. It is synthesized in the nucleus with the participation of the enzyme RNA polymerase. Complementary to the region of DNA where synthesis occurs. Its function is to remove information from DNA and transfer it to the place of protein synthesis - to ribosomes. Makes up 5% of the cell's RNA.
Ribosomal RNA– synthesized in the nucleolus and is part of the ribosomes. Makes up 85% of the cell's RNA.
Transfer RNA– transports amino acids to the site of protein synthesis. It has the shape of a clover leaf and consists of 70-90 nucleotides.
Adenosine triphosphoric acid - ATP
– is a nucleotide consisting of the nitrogenous base adenine, the carbohydrate ribose and three phosphoric acid residues, two of which store a large amount of energy. When one phosphoric acid residue is eliminated, 40 kJ/mol of energy is released. The ability to store such an amount of energy makes ATP its universal source. ATP synthesis occurs mainly in mitochondria.
Table. Functions of nucleotides in the cell.
Table. Comparative characteristics of DNA and RNA.
Thematic assignments.
Part A
A1. The monomers of DNA and RNA are
1) nitrogenous bases
2) phosphate groups
3) amino acids
4) nucleotides
A2. Messenger RNA function:
1) doubling information
2) removing information from DNA
3) transport of amino acids to ribosomes
4) information storage
A3. Indicate the second DNA strand complementary to the first: ATT – HCC – TSH
1) UAA – TGG – AAC
3) UCC – GCC – ACG
2) TAA – CGG – AAC
4) TAA – UGG – UUC
A4. The hypothesis that DNA is the genetic material of the cell is confirmed by:
1) the number of nucleotides in the molecule
2) DNA individuality
3) ratio of nitrogenous bases (A = T, G = C)
4) the ratio of DNA in gametes and somatic cells (1:2)
A5. The DNA molecule is capable of transmitting information thanks to:
1) nucleotide sequences
2) number of nucleotides
3) ability to self-double
4) spiralization of the molecule
A6. In which case is the composition of one of the RNA nucleotides correctly indicated?
1) thymine – ribose – phosphate
2) uracil – deoxyribose – phosphate
3) uracil - ribose - phosphate
4) adenine – deoxyribose – phosphate
Part B
IN 1. Select the features of a DNA molecule
1) Single chain molecule
2) Nucleotides – ATUC
3) Nucleotides – ATGC
4) Carbohydrate – ribose
5) Carbohydrate – deoxyribose
6) Capable of replication
AT 2. Select functions characteristic of RNA molecules of eukaryotic cells
1) distribution of hereditary information
2) transfer of hereditary information to the site of protein synthesis
3) transport of amino acids to the site of protein synthesis
4) initiation of DNA replication
5) formation of ribosome structure
6) storage of hereditary information
Part C
C1. Establishing the structure of DNA allowed us to solve a number of problems. What do you think these problems were and how were they solved as a result of this discovery?
C2. Compare nucleic acids by composition and properties.
A spatial model of the DNA molecule was proposed in 1953 by American researchers, geneticist James Watson (born 1928) and physicist Francis Crick (born 1916). For their outstanding contributions to this discovery, they were awarded the 1962 Nobel Prize in Physiology or Medicine.
Deoxyribonucleic acid (DNA) is a biopolymer whose monomer is a nucleotide. Each nucleotide contains a phosphoric acid residue connected to the sugar deoxyribose, which, in turn, is connected to a nitrogenous base. There are four types of nitrogenous bases in the DNA molecule: adenine, thymine, guanine and cytosine.
The DNA molecule consists of two long chains intertwined in the form of a spiral, most often right-handed. The exception is viruses that contain single-stranded DNA.
Phosphoric acid and sugar, which are part of nucleotides, form the vertical base of the helix. The nitrogenous bases are located perpendicularly and form “bridges” between the helices. The nitrogenous bases of one chain combine with the nitrogenous bases of another chain according to the principle of complementarity, or correspondence.
The principle of complementarity. In a DNA molecule, adenine combines only with thymine, guanine - only with cytosine.
The nitrogenous bases are optimally matched to each other. Adenine and thymine are connected by two hydrogen bonds, guanine and cytosine by three. Therefore, more energy is required to break the guanine-cytosine bond. Thymine and cytosine, which are the same size, are much smaller than adenine and guanine. The thymine-cytosine pair would be too small, the adenine-guanine pair would be too large, and the DNA helix would be bent.
Hydrogen bonds are weak. They are easily torn and just as easily restored. The double helix chains can move apart like a zipper under the action of enzymes or at high temperatures.
5. RNA molecule Ribonucleic acid (RNA)
The ribonucleic acid (RNA) molecule is also a biopolymer, which consists of four types of monomers - nucleotides. Each monomer of an RNA molecule contains a phosphoric acid residue, the sugar ribose and a nitrogenous base. Moreover, the three nitrogenous bases are the same as in DNA - adenine, guanine and cytosine, but instead of thymine, RNA contains uracil, which is similar in structure. RNA is a single-stranded molecule.
The quantitative content of DNA molecules in cells of any species is almost constant, but the amount of RNA can vary significantly.
Types of RNA
Depending on the structure and function performed, three types of RNA are distinguished.
1. Transfer RNA (tRNA). Transfer RNAs are mainly found in the cytoplasm of the cell. They transport amino acids to the site of protein synthesis in the ribosome.
2. Ribosomal RNA (rRNA). Ribosomal RNA binds to certain proteins and forms ribosomes - organelles in which protein synthesis occurs.
3. Messenger RNA (mRNA), or messenger RNA (mRNA). Messenger RNA carries information about protein structure from DNA to the ribosome. Each mRNA molecule corresponds to a specific section of DNA, which encodes the structure of one protein molecule. Therefore, for each of the thousands of proteins that are synthesized in the cell, there is its own special mRNA.
(along with RNA), which are polymers, or more precisely, polynucleotides (monomer - nucleotide).
DNA is responsible for storing and transmitting the genetic code during cell division. It is through DNA molecules that heredity and variability are realized. All types of RNA are synthesized from DNA. Further, various types of RNA jointly ensure the synthesis of cell proteins, i.e., they implement genetic information.
In eukaryotic cells, the vast majority of DNA is found in the nucleus, where it complexes with special proteins to form chromosomes. In prokaryotic cells there is one large circular (or linear) DNA molecule (also in complex with proteins). In addition, eukaryotic cells have their own DNA in mitochondria and chloroplasts.
In the case of DNA, each nucleotide consists of 1) a nitrogenous base, which can be adenine, guanine, cytosine or thymine, 2) deoxyribose, 3) phosphoric acid.
The sequence of nucleotides in a DNA chain determines the primary structure of the molecule. DNA is characterized by a secondary structure of the molecule in the form of a double helix (most often right-handed). In this case, two DNA strands are connected to each other by hydrogen bonds formed between complementary nitrogenous bases.
Adenine is complementary to thymine, and guanine is complementary to cytosine. Two hydrogen bonds are formed between adenine and thymine, and three between guanine and cytosine. Thus, guanine and cytosine are connected to each other a little more tightly (although the hydrogen bonds are weak in principle). The number of bonds is determined by the structural features of the molecules.
Adenine and guanine are purines and consist of two rings. Thymine and cytosine are single-ring pyrimidine bases. Thus, between the backbones (consisting of alternating deoxyribose and phosphoric acid) of two DNA chains, for any pair of nucleotides of different chains, there are always three rings (since a two-ring purine is always complementary only to a certain single-ring pyrimidine). This allows the width between the strands of the DNA molecule to be kept the same throughout (approximately 2.3 nm).
There are approximately 10 nucleotides in one turn of the helix. The length of one nucleotide is approximately 0.34 nm. The length of DNA molecules is usually enormous, exceeding millions of nucleotides. Therefore, in order to fit more compactly into the cell nucleus, DNA undergoes varying degrees of “supercoiling.”
When reading information from DNA (that is, synthesizing RNA on it, this process is called transcription) despiralization occurs (the reverse process of spiralization), the two chains diverge under the action of a special enzyme. Hydrogen bonds are weak, so separation and subsequent cross-linking of chains occurs with little energy consumption. RNA is synthesized from DNA according to the same principle of complementarity. Only instead of thymine in RNA, uracil is complementary to adenine.
The genetic code, written on DNA molecules, consists of triplets (sequences of three nucleotides) that represent one amino acid (protein monomer). However, most DNA does not code for protein. The significance of such parts of the molecule varies and is largely not fully understood.
Before a cell divides, the amount of DNA always doubles. This process is called replication. It is semi-conservative in nature: the chains of one DNA molecule diverge, and each one is completed with its own new complementary chain. As a result, from one double-stranded DNA molecule two double-stranded DNAs are obtained, identical to the first.
In DNA, polynucleotide chains are multidirectional, i.e., where one chain has a 5" end (a phosphoric acid residue is attached to the fifth carbon atom of deoxyribose), the other has a 3" end (carbon free from phosphoric acid).
During replication and transcription, synthesis always proceeds in the direction from the 5" end to the 3" end, since new nucleotides can only attach to the free 3" carbon atom.
The structure and role of DNA as a substance responsible for hereditary information was clarified in the 40-50s of the 20th century. In 1953, D. Watson and F. Crick determined the double-stranded structure of DNA. Previously, E. Chargaff found that in DNA the amount of thymine always corresponds to adenine, and the amount of guanine to cytosine.
1. What types of nucleic acids exist? What are nucleic acid monomers?
There are two types of nucleic acids: c) RNA, d) DNA.
The monomers of nucleic acids are: f) nucleotides.
2. Describe the structure of a nucleotide. How can nucleotides join together in a DNA molecule?
A nucleotide consists of a nitrogenous base, a five-carbon sugar (pentose) and a phosphoric acid residue. The DNA nucleotide contains one of four nitrogenous bases (adenine, guanine, cytosine or thymine), and the five-carbon sugar is deoxyribose. In an RNA nucleotide, the nitrogenous base is adenine, guanine, cytosine, or uracil, and the five-carbon sugar is ribose.
A DNA molecule consists of two polynucleotide chains. The nucleotides in each chain are connected to each other by covalent bonds. These bonds are formed between the phosphoric acid residue of one nucleotide and the pentose of another nucleotide. Paired nucleotides of opposite DNA strands are connected by hydrogen bonds, with two hydrogen bonds formed between adenine and thymine, and three between guanine and cytosine. This matching of paired nucleotides is called complementarity.
3. The nucleotide sequence of one of the DNA chains has been established: CTGAGTTCA. Determine the order of the nucleotides of the complementary chain.
In a DNA molecule, adenine (A) is complementary to thymine (T), and guanine (G) is complementary to cytosine (C), therefore the order of the nucleotides of the complementary DNA chain will be as follows: GACTCAAHT.
4. Describe the spatial structure of the DNA molecule.
The DNA molecule consists of two polynucleotide chains twisted around a common axis and is a double helix with a diameter of about 2 nm (like a spiral staircase). Each turn of the helix contains 10 base pairs and is 3.4 nm long. Opposite DNA strands complement each other complementarily, since the nucleotides of these chains form pairs (A and T, G and C). Hydrogen bonds occur between paired nucleotides, stabilizing the DNA double helix.
5. What types of RNA are found in the cell? Compare them by their functions, structural features and percentage of the total amount of RNA in the cell.
The cell contains three types of RNA: ribosomal (rRNA), transport (tRNA) and informational, or matrix (mRNA, mRNA). The functions of all types of RNA are associated with protein synthesis processes.
rRNA molecules perform a structural function. In combination with special proteins, they acquire a certain spatial configuration and form ribosomes (or rather, ribosomal subunits), on which proteins are synthesized from amino acids.
Transfer RNAs carry out the transfer of amino acids to ribosomes and participate in the process of protein synthesis. tRNA molecules are relatively small (on average they consist of 80 nucleotides); thanks to intramolecular hydrogen bonds, they have a specific spatial structure, reminiscent of a clover leaf.
Messenger or messenger RNA (mRNA, mRNA) are the most heterogeneous in size and structure. They contain information about the structure of certain proteins and serve as templates during the synthesis of these proteins on ribosomes.
Ribosomal RNAs make up about 80% of all cell RNAs, transport RNAs - about 15%, informational RNAs - 3-5%.
6. Compare DNA and RNA according to various characteristics. Identify their similarities and differences.
Similarities:
● They are organic substances, biopolymers, and belong to nucleic acids.
● Constructed from nucleotides, each of them contains a nitrogenous base, a pentose and a phosphoric acid residue. The nitrogenous bases adenine (A), guanine (G) and cytosine (C) are found in both DNA and RNA nucleotides.
● Molecules are formed by carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and phosphorus (P) atoms.
Differences:
● DNA nucleotides contain a five-carbon sugar deoxyribose residue, while RNA nucleotides contain a ribose residue. The nitrogenous base thymine (T) can only be found in DNA nucleotides, and uracil (U) can only be found in RNA nucleotides.
● The DNA molecule is double-stranded (with rare exceptions) and looks like a double helix. RNA molecules are usually single-stranded and can have different spatial configurations. RNA polynucleotide chains are much shorter than DNA chains.
● In eukaryotic cells, most of the DNA is contained in the nucleus (only mitochondria and chloroplasts have their own small DNA molecules). RNA molecules are found not only in the nucleus, but also in the cytoplasm of cells - as part of some organelles (ribosomes, mitochondria, chloroplasts), and in the hyaloplasm.
● In a cell, DNA ensures the storage of hereditary information (i.e. information about the structure of proteins) and its transmission to daughter cells during the process of division. RNA molecules ensure the implementation of hereditary information by participating in the process of protein biosynthesis on ribosomes.
And (or) other significant features.
7. A fragment of a DNA molecule contains 126 adenyl nucleotides (A), which is 18% of the total number of nucleotides in this fragment. What is the length of this DNA fragment and how many cytidyl nucleotides (C) does it contain?
126 nucleotides make up 18% of all nucleotides in a given DNA fragment. This means that the total number of nucleotides is: 126: 18% × 100% = 700 nucleotides (or 350 base pairs).
One turn of the DNA double helix contains 10 base pairs and is 3.4 nm long. Therefore, one pair of nucleotides occupies a DNA region 0.34 nm long. A DNA fragment containing 350 nucleotide pairs has a length of: 350 × 0.34 nm = 119 nm.
In a double-stranded DNA molecule A = T, G = C. This means A = T = 126 nucleotides.
The sum of G + C is: 700 – 126 – 126 = 448 nucleotides. G = C = 448: 2 = 224 nucleotides.
Answer: The DNA fragment is 119 nm long and contains 224 cytidyl (C) nucleotides.
8. A researcher has three DNA molecules of the same length. It is known that the content of thymidyl nucleotides (T) in the first sample is 20% of the total number of nucleotides, in the second - 36%, in the third - 8%. He began heating these DNA samples, gradually increasing the temperature. In this case, the complementary chains were separated from each other - the so-called DNA melting. Which sample started to melt first, and which one melted last? Why?
DNA melting occurs due to the breaking of hydrogen bonds between complementary nucleotides. Two hydrogen bonds are formed between adenine and thymine, and three between guanine and cytosine. The higher the content of G–C pairs in a DNA fragment, the more hydrogen bonds there are in its composition, and the more energy will be required to destroy them. Conversely, the more A–T pairs a DNA fragment contains, the less energy will be needed for melting.
Therefore, the second sample will melt first (it contains the most thymine, and therefore the A–T pairs), then the first, and lastly the third (with the least thymine content).
Nucleic acids.
Nucleic acids are natural high-molecular compounds (polynucleotides) that play a huge role in the storage and transmission of hereditary information in living organizations.
There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These biopolymers are made up of monomers called nucleotides.
The main location of DNA is the cell nucleus. DNA is also found in some organelles (plastids, mitochondria, centrioles). RNAs are found in the nucleoli, ribosomes and cytoplasm of cells.
The DNA molecule is a structure consisting of two strands, which are connected to each other along their entire length by hydrogen bonds. This structure is called a double helix. Hydrogen bonds occur between the purine base of one chain and the pyrimidine base of another chain. These bases make up complementary pairs (from the Latin complementum - addition).
Characteristics of DNA.
1. DNA (deoxyribonucleic acid) is a linear polymer in the form of a double helix formed by a pair of antiparallel complementary chains. The monomers of DNA are nucleotides.
2. DNA nucleotides consist of purine (A - adenine or G - guanine) or pyrimidine (T - thymine or C - cytosine) nitrogenous bases, a five-carbon sugar - deoxyribose - and a phosphate group.
3. The DNA molecule has the following parameters: the width of the helix is about 2 nm, the pitch, or full turn, of the helix is 3.4 nm. One step contains 10 complementary nucleotides.
4. Nucleotides in a DNA molecule face each other with nitrogenous bases and are united in pairs in accordance with the rules of complementarity: thymine is located opposite adenine, and cytosine is located opposite guanine. The AL1 pair is connected by two hydrogen bonds, and the G-C pair is connected by three.
5. The backbone of DNA chains is formed by sugar phosphate residues.
6. DNA replication is the process of self-duplication of a DNA molecule, carried out under the control of enzymes.
On each of the chains formed after the rupture of hydrogen bonds, a daughter DNA strand is synthesized with the participation of DNA polymerase. The material for synthesis is free nucleoside phosphates present in the cytoplasm of cells.
7. The synthesis of daughter molecules on neighboring chains occurs at different rates. On one chain a new molecule is assembled continuously, on the other - with some lag and in fragments. After the process is completed, fragments of new DNA molecules are stitched together by the enzyme DNA ligase. So from one DNA molecule two DNA molecules arise, which are exact copies of each other and the mother molecule. This method of replication is called semi-conservative.
8. The biological meaning of replication lies in the accurate transfer of hereditary information from the mother molecule to the daughter molecules, which occurs during the division of somatic cells.
Characteristics of RNA.
RNA is a linear polymer consisting of a single chain of nucleotides. In RNA, the thymine nucleotide is replaced by uracil (U). RNA nucleotides contain the five-carbon sugar ribose, one of the four nitrogenous bases, and a phosphoric acid residue.
Types of RNA:
Matrix, or messenger, RNA is synthesized in the nucleus with the participation of the enzyme RNA polymerase. Complementary to the region of DNA where synthesis occurs. Makes up 5% of the cell's RNA;
Ribosomal RNA is synthesized in the nucleolus and is part of ribosomes. Makes up 85% of the cell's RNA;
Transfer RNA (more than 40 types) - transports amino acids to the site of protein synthesis. It has a cloverleaf structure and consists of 70-90 nucleotides.